The present invention relates to a brake system for motor vehicles comprising a master cylinder to which wheel brake cylinders are connectable, a first piston which is coupled to a brake pedal by way of a push rod that transmits actuating forces, a second piston which actuates the master cylinder, a third piston which can be actuated by the first piston and is movable into a force-transmitting connection with the second piston, with at least one elastic element forming a pedal travel simulator that imparts a pleasant pedal feeling to the operator in a ‘brake-by-wire’ operating mode, with a means to couple the movements of the first piston and the third piston being activated depending on the displacement travel of the third piston in a housing, with a space between the second and the third pistons to which hydraulic pressure is applicable, wherein a pressurization of the space loads the second and the third pistons in opposite directions, as well as a pressure supplying module, which allows both filling the space with pressure fluid and evacuating it.
‘Brake-by-wire’ brake systems are employed in motor vehicle technology at an increasing rate. In the brake systems, the brake can be actuated ‘independently’ in response to electronic signals without any action on the part of the driver. The electronic signals can be output by an electronic stability program ESP or a collision avoidance system ACC, for example. When an independent actuation of this type is superposed on an actuation by the driver, the driver of the motor vehicle notices a reaction in the brake pedal. This reactive effect on the brake pedal can be surprising and unpleasant for the driver so that, in a critical situation in traffic, the driver will not apply the brake pedal to an extent that complies with this situation because the reaction to the brake pedal that is due to the independent actuation of the brake is irritating him.
DE 10 2004 025 638 A1 discloses a brake system of the type mentioned hereinabove. An ‘independent actuation’ of the prior-art brake system or pressurization of the space is executed in the ‘brake-by-wire’ operating mode by the electrically controllable pressure-supplying module by means of a likewise electrically operable valve device, which opens or closes a hydraulic connection between the space and the pressure-supplying module, respectively. The fact is considered disadvantageous in the prior-art brake system that boosting of the actuating force generated by the operator is possible only when the electronic unit and the energy supply (battery) provided in the vehicle are intact.
In view of the above, an object of the invention is to disclose a brake system of the type initially referred to, wherein the actuating force is boosted even if the electronic control unit or the electric energy supply fails.